Process of producing 5xxx series aluminum alloys with high mechanical, properties through twin-roll casting

ABSTRACT

Present invention concerns a method for making fine-grained, formable aluminium alloy strips containing (by weight) essentially between 0.5-6.5% Mg, 0-0.50% Si, 0-0.60% Fe, 0-1.2% Mn, 0-0.50% Cr, by twin-roll casting to a thickness ranging between 4 and 6.5 mm and cold rolling the strips to an intermediate gauge and reroll annealing the intermediate gauge material. The reroll-annealed material is then cold rolled to a final sheet gauge followed by a final recrystallizing or back annealing. The combination of controlled casting parameters, controlled amounts of Fe, Si, Mn, Cr and Mg and reroll and final annealing temperatures results in an improved sheet product in terms of finer grain size, higher elongation and formability, age softening and better corrosion resistance. Homogenization may be performed at  420 ° C. to  550 ° C. for a period of 4 to 15 hours and recrystallization is performed at 280° C. to 375° C. for a period of not less than 4 hours and not more than 8 hours.

FIELD OF THE INVENTION

[0001] The present invention is directed to a process for the productionof 5XXX series aluminum alloys, from continuously cast aluminum stripsuitable for using in automotive and transportation applications,exhibiting superior or at least equivalent mechanical, microstructural,corrosion resistance and formability properties than those of similaralloys produced by the DC casting routes.

BACKGROUND OF THE INVENTION

[0002] Weight reduction is a primary goal in transportation industryincluding especially automotive and marine technologies not only toimprove fuel economy, but also to improve performance, safety &durability and to reduce emissions. The use of light-weight materials isone of the most promising ways of achieving this goal and aluminumcurrently offers the greatest potential for cost-effective weightsavings in automotive body structures. Aluminum has the advantage overcompetitive materials due to a very attractive combination of density,strength, formability and ease of recycling. Although there is extensiveuse of aluminum in die-cast parts, wrought aluminum has so far beenconfined to relatively few applications.

[0003] Aluminum, with only one-third the density of steel and a betterstrength-to-density ratio, could provide weight reductions of nearly 50%when used in place of steel in automotive sheet applications. Inaddition, the fabrication, assembling and recycling practices foraluminum components are largely the same as those employed for steel,making aluminum particularly favorable for these applications. The majorbarrier to the widespread use of aluminum in high-volume automotiveapplications has not been its technical shortcomings, but its high cost,which is roughly four to five times that of sheet steel. Production ofaluminum sheet by the strip casting route rather than by theconventional DC casting and hot mill method offers an opportunity tosubstantially reduce this cost, which could lead to an increase in itsuse. While block and belt casting have received some attention, thepotential of Twin Roll Casting (TRC) process in the manufacture ofaluminum sheet for automotive structural applications has so far beenoverlooked. Recently, there has been a growing interest in using TRC asa method to produce low-cost/high-quality aluminum sheet for suchapplications. Production of aluminum sheets having Mg content between3.0 and 5.0% by direct chill (DC) casting method was proposed by Yamadaet. al in U.S. Pat. No. 4,826,737. Mg content of the alloy was varyingbetween 0,5 and 3,8 in another application proposed by Wong in U.S. Pat.No. 3,661,657. The production process of that invention, however, isbased on DC casting method, as well.

[0004] In spite of the successful use of 5XXX series aluminum alloys instamping, press shop and other forming operations of DC cast aluminumingots, economic and energy considerations would favor the manufactureof the these alloys by continuous strip casting. In this process themolten metal is cast and solidified into a thin strip of 6 mm or less inthickness so that subsequent rolling is reduced and costly step of hotrolling is eliminated. Compared to the previous applications, such as inU.S. Pat. No. 4,235,646 by Neufeld et al., wherein the casting and finalgauge of the end-product is limited to certain thicknesses, presentinvention allows production of the as-cast strip between 3 to 7 mm andvarious thickness of the end products having soft and H2X, H3X tempers.

[0005] Properties required of aluminum sheet for automotive applicationsare high strength, good formability, weldability and corrosionresistance and are met largely by a number of Al—Mg (5XXX series) alloysthat are the subject of the present invention. Al—Mg alloys have beenused to form auto body trim parts, doors, hood, truck vessels, fueltanks, pressurized air vessels of trucks, truck tanks, ships and theirinfrastructure and superstructure, dump truck bodies, cryogenic vesselsand LPG tanks.

INDUSTRIAL APPLICABILITY

[0006] Present invention relates to an aluminum alloy with highmechanical properties and method for its manufacture. Therefore, theinvention has industrial applicability in the field of processingmetals.

SUMMARY OF THE INVENTION

[0007] In accordance with the objective of the present invention, awrought Al—Mg alloy sheet produced through Twin Roll Casting techniqueand accordingly processed to its final gauge aiming to provide superioror at least equivalent microstructural, mechanical, corrosion resistanceand formability properties compared to their counter parts produced byDC casting and mill route is provided. The sheet product containsessentally between 0.5-6.5% Mg, 0, 0-0.50% Si, 0, 0-0.60% Fe, 0, 0-1.2%Mn, 0, 0-0.50% Cr by weight.

BRIEF DESCRIPTION OF FIGURES

[0008]FIG. 1, is the flowchart showing the casting process according tothe present invention, FIG. 2-a, is the flowchart showing cold rollingand annealing processes according to an embodiment of the inventionwhere no homogenization treatment is applied.

[0009]FIG. 2-b, is the flowchart showing cold rolling and annealingprocesses according to another embodiment of the invention where ahomogenization treatment is applied immediately after casting.

[0010]FIG. 2-c, is the flowchart showing cold rolling and annealingprocesses according to a further embodiment of the invention where ahomogenization treatment is applied following a cold rolling process.

[0011]FIG. 3, shows photographs of very fine discrete particles near thesurface of (a) Alloy A, (c) Alloy B and (e) Alloy C in as-cast materialswhere interdendritic network of primary phases in the interior of thesame strips are shown for (b) Alloy A, (d) Alloy B and (f) Alloy C.

[0012]FIG. 4, shows photographs of the staining observed after 96 hrsexposure to salt spray, on following samples (a) DC-cast A, (b) DC-castB (c) strip cast A w/homog, (d) strip cast A w/o homog., (e) strip castB w/homog., (f) strip cast B w/o homog. and (g) strip cast C w/homog.

DETAILED DESCRIPTION OF THE INVENTION

[0013] The present invention is directed to a process for thepreparation of 5XXX series aluminum alloys, from continuously castaluminum strip suitable for using in automotive and other transportationapplications, exhibiting superior or at least equivalent mechanical,microstructural, corrosion resistance and formability properties thanthose of similar alloys produced by the DC casting routes.

[0014] When compared to the DC casting and hot mill method, Twin RollCasting technology has a relatively shorter process route (FIG. 1) withless working of the metal. As is claimed in number of publications, theprocessing route, that comprises DC casting and successive hot milloperations, is envisioned as a unique way for the production of 5XXXseries aluminum alloys in different thickness.

[0015] As will hereinafter be illustrated, it has been determined thatin the strip casting of 5XXX series aluminum alloys, control of thecasting speed, melt temperature, cast sheet thickness, alloy compositionand critical positioning of the caster tip (that is a ceramic structuredelivering the molten metal in between the caster rolls) with respect tothe centerine of the caster rolls, called set-back, render to obtainappropriate as-cast sheet tailored with optimum microstructural andmechanical properties. These features are the first essential and basicrequirements of the material that will be processed in certain routes toachieve desired mechanical performance in the final product Control ofthe cold roll, recovery and recrystallization anneal will result intwin-roll cast aluminum sheet exhibiting superior or at least equivalentmicrostructural, mechanical, corrosion resistance and formabilityproperties than those of sheets produced through DC casting andsubsequent hot mill operations. Chemical composition of the alloys thatwill be presented in the body of this application is given in Table 1.TABLE 1 The chemical composition of the alloys. ALLOY Si, % Fe, % Cu, %Mn, % Mg, % Cr, % Al, % A 0.125 0.286 0.030 0.059 2.536 0.163 96.71 B0.177 0.328 0.043 0.365 4.210 0.153 94.63 C 0.165 0.307 0.037 0.2863.062 0.067 95.98

[0016] The microstructure of the as-cast strip is characterized by adendritic structure of the aluminum solid solution with the primaryphases marking the dendrite boundaries. The interdendritic network ofthe primary phases is well defined in the interior of the strip whereasit is deteriorated somewhat and is replaced by a very fine dispersion ofdiscrete particles near the surface. The dendritc structure is quiteuniform across the thickness of all strip samples. The microstructuralfeatures, i.e. dendrite arm spacing, primary particle size, arerelatively smaller at the surface and coarsened only slightly whenapproaching the center of the strip. However, such variations across thethickness of the 5XXX as-cast strip are not as prominent as they are insome common foil and finstock alloys. The extent of supersaturation waslargest in the alloys having high Mg content.

[0017] The structure near the surface reveal the characteristicappearance of a deformed material in strip samples. Grains appeared moreand more elongated and flattened, and the intermetallic particles moreand more oriented near the surface, suggesting that the surface hasexperienced substantial deformation in the caster roll gap (FIG. 3).Evidence of surface deformation is more prominent in the case of Alloy Aand C. The smaller casting gauge selected for the latter alloy isbelieved to be responsible. Even a small, only as much as 0.5 mm,decrease in the casting gauge apparently has a profound effect onsurface deformation. The central part of the strip, on the other hand,reveals a more or less equiaxed, uniform grain structure and a randomdispersion of the primary phases. It is thus concluded that thedeformation during casting was limited to the surface layers and has hadlittle effect, if any, in the central part of the strip.

[0018] Three types of segregation patterns were identified in theas-cast strip. Alloy B strip shows traces of surface segregation witheutectic rich bands, running in the casting direction, only along theedges of the cast strip. These bands, however, do not penetrate deepinto the strip and were estimated to be less than 5 μm deep in the mostsevere cases. It should be noted that this edge region of any cast stripis removed by an edge trimming operation during cold rolling. There ishardly any evidence of surface segregation in Alloy A and Alloy C strip.Some intergranular and centerline segregation is also observed in allstrip samples. The latter occasionally produced channels of solute-richmaterial and is most prominent in the Alloy B. Although centerlinesegregation is believed to have negative effects on mechanicalproperties—at least to some extent—, these effects may be minimized withappropriate casting parameters.

[0019] It is concluded from the metallographic analysis of the sheetsamples that the surface and intergranular segregation patterns have notsurvived to the final gauge. There was no evidence of surface andintergranular segregation patterns at the final gauge except for sometraces of centerline segregation. The solute-rich eutectic regions atthe centerline have apparently transformed into clusters of discreteparticles which have largely spherodized.

[0020] To effect the most advantageous improvement in thethrough-thickness grain structure along with mechanical properties, suchas lower yield strength, higher elongation and higher UTS (ultimatetensile strength), at the final gauge and soft temper of the material,homogenization treatment should be applied after a cold rolling pass.

[0021] TWQ different processing routes (FIG. 2a and 2 b-2 c)whereinexistence of homogenization treatment determining this difference, canbe applied to the as-cast strip. Homogenization treatment of the stripis performed at the temperature between 420° and 550° C. for a period oftime, preferably not less than 4 hours and not more than 15 hours.Homogenization treatment is carried out in an inert gas atmosphere ofthe batch annealing furnaces. In the processing route, in which as-caststrip is subjected to homogenization treatment, two different methodsare employed depending on when it is applied: While homogenizationtreatment can be performed at the casting gauge (FIG. 2b), it can alsobe applied after a first cold rolling pass that must provide at least25% reduction in the thickness of as-cast strip (FIG. 2c). The grainstructure at the final gauge is improved in a profound way when caststrip is homogenized after a cold rolling pass . The homogenized stripmaterial is subjected to cold rolling wherein no recrystallizationanneal is applied until a critical gauge. Recrystallization anneal atthis critical gauge and subsequently applied predetermined amount ofcold woridng and application of back annealing procedure at the finalgauge, as a final step in the production cycle, provides strain hardenedand partially annealed, H2X tempers, final products including, H22, H24,H26.

[0022] Recrystallization at the final gauge in practice of the presentinvention is in the range of 280° to about 375° C., preferably for about2 to 8 hours. Prior to the recrystallization anneal applied to obtain 0temper material at the final gauge, as-cast strip is cold rolled to thefinal gauge without applying any intermediate annealing in processingroute as shown in FIG. 2a. Similarly, the material that is subjected tothe homogenization treatment, according to the processing routes shownin FIG. 2b and 2 c, is also cold rolled to the aimed gauge withoutapplication of intermediate annealing. Intermediate annealing applied atthe predetermined stage of the processing route is also considered asrecrystallization anneal and carried out by applying the sametemperature and time combination.

[0023] Back annealing temperatures to render H2X temper materials at thefinal gauge are in the range of 130°-250° C., preferably 2 to 8 hours.

[0024] Production routes of the final products having mechanicalproperties of H3X tempers principally follow the similar processingroute of H2X tempers. After heating at the recrystallization temperaturefor the above mentioned prescribed time period, coil is subjected tofinal cold roll of at least 20% to attain aimed final gauge. To achievean optimum mechanical property requirement of H3X tempers, this annealis effected at a temperature between about 100 and 190° C. for about 2to 6 hours.

[0025] Aforementioned processing routes of the present invention, thatcomprises a homogenization treatment, can be altered in the way ofeliminating homogenization treatment by keeping the other down streamoperations in effect (FIG. 2-a). Absence of homogenization step in theprocessing route does not create substantial changes in the mechanicalproperties, microscopic features, corrosion resistance and formabilityproperties of the Twin Roll Cast 5XXX series alloys at different tempers(soft, strain hardened, or H2X/H3X) of final gauges. This radicalalteration in the processing route provides significant contribution tothe economical aspect of Twin Roll Cast 5XXX series alloys, as well.

[0026] Superior corrosion resistance of the Twin-roll cast 5XXX seriesalloys were tested in the same testing environment with their DC-castcounterparts. Corrosion test samples of Alloy A, Alloy B and Alloy Chaving 10 cm² test areas were exposed to a salt spray test (ASTM B-117)for 96 hrs. with their counter parts produced through DC casting route.After exposure of 96 hrs to salt spray, Twin roll cast Alloy A, Alloy Band Alloy C samples did not develop corrosion products on their surface(FIG. 4).

EXAMPLE I

[0027] Aluminum alloy melt having melt composition given in Table 1-a.was prepared having the following composition: TABLE 1-a Chemicalcomposition of Alloy A. Alloy Si Fe Cu Mn Mg Cr Al A 0.125 0.286 0.0300.059 2.536 0.163 96.71

[0028] The melt was thoroughly mixed and then in-line refining wasapplied with chlorine and inert gas mixture to reduce the hydrogencontent of the melt below 0.20 ml/100 grams of metal and its inclusioncontent. Hydrogen gas level was reduced to 0.12 mV/100 grams of metal.The metal was cast in the form of 1750 mm wide, 5.35 mm thick industrialsize coils. Prior to cold rolling operation, one of the cast coil washomogenized in inert atmosphere. Coil was cold rolled in successivepasses to its final gauge of 1 mm without any intermediaterecrystallization anneal. Soft temper of the material was achieved withfinal a recrystallization anneal. Another coil was processed through thesame processing route, with the exception of homogenization treatment,to the final gauge of 1 mm. Tensile tests were performed on the samplesprepared in three different directions with respect to the rollingdirection namely; parallel, transverse and 45° to the rolling direction.Strain hardening exponent of the samples (n), in three differentdirections, were calculated from stress-strain diagram of the samplesbetween the strain of 5 to 18%. First indication, to evaluate formingability of the material was obtained through Erichsen Cup index valuesof the materials. Identical tests and characterization methods werecarried out on the other coil that was not homogenized. Mechanical testresults of these two coils were shown in Table 2. TABLE 2 Mechanicalproperties of Alloy A. Erichsen, Alloy Process mm σ_(y), MPa σ_(f), MPaElongation, % n A With 9.8  0°/96 204 22 0.26 homog. 45°/94 197 24 0.2790°/96 198 24 0.26 Without 9.6  0°/101 207 22 0.28 homog. 45°/102 205 270.27 90°/104 206 24 0.26

EXAMPLE II

[0029] An aluminum alloy in accordance with the present invention,designated “B” consisted essentially of the elements stated in Table 3.This alloy was cast into 5.45 mm thick strip. TABLE 3 Chemicalcomposition of Alloy B. Alloy Si Fe Cu Mn Mg Cr Al B 0.177 0.328 0.0430.365 4.210 0.153 94.63

[0030] After necessary melt treatment operations that were carried outprior to the casting 1750 mm wide sheet was cast with the casting speedof 110 mm/min. 5 Processes with and without homogenization treatmentswere applied to these industrial sized (8 ton) coils, all of which weregiven a recrystallization anneal at the final gauge of 1 mm after a coldrolling schedule. Coils were not subjected to any intermediate anneal atany stage of the down stream process.

[0031] Mechanical properties of the material at the final gauge havingsoft temper were presented in Table 4. Mechanical properties of thismaterial did not exhibit significant difference depending on theprocessing route applied. TABLE 4 Mechanical properties of alloy B at 1mm. Erichsen, Alloy Process mm σ_(y), MPa σ_(f), MPa Elongation, % n⁽¹⁾B With 9.3  0°/146 281 22 0.28 homog. 45°/142 271 27 0.27 90°/146 269 220.26 Without 9.1  0°/145 270 26 0.29 homog. 45°/144 268 29 0.29 90°/147272 22 0.29

[0032] The alloys and examples embedded above are given solely forexemplifying the invention and therefore the invention shall not beconsidered as being limited with the same.

1-10. cancelled.
 11. A process capable for producing 5XXX seriesaluminum alloys through twin-roll casting comprising the steps of: (a)providing the twin-roll caster with a melt composition comprisingbetween 0.5-6.5% Mg, 0-0.50% Si, 0-0.60% Fe, 0-1.2% Mn and 0-0.50% Cr byweight, where the temperature of the melt is kept essentially between690° C. and 715° C., (b) operating the twin-roll caster continuouslyunder critical conditions in which the casting speed is varied between0.8 m/min and 1.5 m/min, (c) maintaining a set-back distance essentiallybetween 48 mm and 65 mm, where set-back is the critical distance betweenthe center lines of caster rolls and exit side of the molten metal fromthe caster tip, (d) casting a strip with uniform thickness adjustedwithin the range of 3-7 mm., (e) applying a first cold rolling treatmentto as-cast strip, (f) allowing recrystallization at the critical gaugeessentially in the range of 280° C. to about 375° C., for a period ofnot less than 4 hours and not more than 8 hours, and (g) applying asecond cold rolling treatment to said strip following therecrystallization step.
 12. A process according to claim 11, wherein theprocess further comprises an homogenization step applied to said as-caststrip, prior to applying the first cold rolling treatment of step e), ata temperature ranging between 420° C. to 550° C. for a period of 4 to 15hours,
 13. A process according to claim 12, wherein the process furthercomprises the step of applying a cold rolling treatment to as-cast stripprior to applying said homogenization treatment.
 14. A process as setforth in any of the claims 11 to 13, further comprising a finalannealing treatment suitable for producing H2X tempers, wherein saidfinal annealing treatment is applied essentially within temperaturerange of 130°-250° C., for a period of not less than 100 minutes and notmore than 8 hours.
 15. A process as set forth in any of the claims 11 to13, further comprising a final annealing treatment suitable forproducing H3X tempers, wherein said final annealing treatment is appliedessentially within the temperature range of 100°-190° C. for a period ofabout 2 hours to 6 hours.
 16. A process capable for producing 0 tempersof 5XXX series aluminum alloys through twin-roll casting comprising thesteps of: (a) providing the twin-roll caster with a melt compositioncomprising between 0.5-6.5 % Mg, 0-0.50% Si, 0-0.60% Fe, 0-1.2% Mn and0-0.50% Cr by weight, where the temperature of the melt is keptessentially between 690° C. and 715° C., (b) operating the twin-rollcaster continuously under critical conditions in which the casting speedis varied between 0.8 m/min and 1.5 m/min, (c) maintaining a set-backdistance essentially between 48 mm and 65 mm, where set-back is thecritical distance between the center lines of caster rolls and exit sideof the molten metal from the caster tip, (d) casting a strip withuniform thickness adjusted within the range of 3-7 mm., (e) applying acold rolling treatment to as-cast strip and allowing recrystallizationat the final gauge essentially in the range of 280° C. to about 375° C.,for a period of not less than 4 hours and not more than 8 hours.
 17. Aprocess as set forth in claim 16 suitable for producing 0 tempers,wherein the process further comprises the step of applying ahomogenization treatment to as-cast strip, prior to applying said thecold rolling treatment of step (e), at a temperature ranging between420° C. to 550° C. for a period of 4 to 15 hours.
 18. A process as setforth in claim 17 suitable for producing 0 tempers, wherein the processfurther comprises the step of applying a first cold rolling treatment toas-cast strip before applying said homogenization treatment.